The present invention relates to cellular wireless communication networks also known as cellular wireless communication systems. More particularly it relates to a method and apparatus for preserving call-state information to avoid service interruptions for stable calls.
A typical cellular wireless communication systems is made up of a plurality of cells each occupying a separate geographical area. Each cell usually includes a cell site having known hardware necessary for providing wireless communication coverage to a plurality of wireless terminals within the cell. Examples of such hardware can include, but is not limited to, radio frequency transmitters and receivers, antenna systems, interface equipment and power sources.
The cell site typically communicates with one or more active processes having application processors which handle access system resources such as radios, channels and the like for the cell. Software applications, known as Radio Control Software (RCS), running on these application processors manage the associated call and maintenance traffic to, from, and within the cell. Several cell sites typically communicate with a Mobile Switching Center (MSC) which switches cellular calls to wired central offices to enable mobile terminals to communicate with phones over the Public Switched Telephone Network (PSTN).
The PSTN offers users the benefit of high availability of service which means call service interruptions are rare. Interruptions in wireless communication service can be created when hardware and/or software fails or is taken off line for repairs, maintenance, updates, etc. As the use of wireless systems grows and the capacities of wireless systems increase, end users and service providers alike desire improvements in service availability.
In an effort to achieve high availability, the processes are paired in an active/standby arrangement using an active RCS application processor mated to a standby RCS application processor. When a fault occurs on the active process, or maintenance is to be performed, the standby process is elevated to the active role in an effort to continue providing service. It is desirable to not drop calls during this procedure.
Conventional cellular wireless communication systems using mated processor pairs have provided high availability by using specialized fault tolerant hardware. These specialized application processors are tightly coupled, connected together by an update bus which updates the memories of the active and standby processors. It is desirable to reduce hardware costs by using general purpose commercial processors without specialized fault tolerant hardware such as the update bus.
Typically, when the active process is interrupted in the high availability architecture system, a stable clear is performed to clear the stable calls being handled by the active process and the active application is relocated to another processor. A stable call is a call that is not waiting for any internal messages. Calls being setup or in hard handover between cell cites are transient calls. For CDMA with an average 100 second call hold time approximately 1 per cent of calls become transient every second. Not only are stable calls dropped, but often the subtending cells must reboot after being disconnected from the application processor and this procedure adds to the recovery duration of the dropped calls. It is, therefore also desirable to reduce the recovery time for any calls which are dropped.